The present invention relates generally to portable electric power tools and particularly to control circuits for tools such as electrically powered staplers or nailers.
Electrically powered staplers or nailers typically employ a solenoid having a reciprocable armature which is operable to eject a staple or nail from the device and drive it into an article. When the winding or coil of the solenoid is energized by a current impulse of sufficient magnitude, the magnetic field created thereby causes the armature to move axially against the resistance of a tension spring with the force required to eject and drive the staple or nail. The stapler/nailer also typically includes a spring loaded trigger which when depressed or squeezed by the operator actuates a trigger or firing circuit controlling the energization of the solenoid.
Due to cost and other manufacturing considerations, it is highly desirable to use the same trigger switches in tools such as power staplers or nailers as are used in other types of power tools. However, the current levels required to operate the solenoid in a power stapler or nailer are substantially greater than the current drawn by a conventional motor-driven power tool such as a drill or rotary saw. Accordingly, since the physical size of such trigger switches is quite small, it has been the practice in the industry to use the trigger switch for a power stapler or nailer to control only the switching of the semiconductor circuit component which controls the energization of the solenoid, so that the trigger switch does not carry the full current loads required to operate the solenoid. An example of this type of circuit design is shown in U.S. Pat. No. 4,183,454, issued Jan. 15, 1980, to Barrett et al.
Since the solenoids for this application are capable of operating within one half cycle of the AC power waveform to drive a staple or nail, the most common circuit element used to control the energization of the solenoid is a silicon controlled rectifier (SCR), also known as a reverse blocking triode thyristor. Although the reliability of SCR's are well known by those skilled in the art, it is nevertheless still possible for an inadvertent misfiring of the fastening device to occur. For example, internal heating of the SCR may cause the SCR to "break down", thereby permitting a current flow through the solenoid coil of operating magnitude. Additionally, power surges in the supply line, such as when a fluorescent light is switched on, may also cause the SCR to suddenly conduct without actuation of the stapler/nailer trigger by the operator. Furthermore, if the particular SCR does not meet the manufacturer's specifications, the forward blocking current or voltage characteristics may be such as to permit the SCR to inadvertently conduct under some conditions.
Accordingly, it is a principal object of the present invention to provide a novel firing circuit for a portable electrically powered fastening device of an economical design which will isolate the solenoid drive of a device from the line voltage powering the device except when the trigger is actuated.
It is a more specific object of the present invention to provide a firing circuit which includes a manually operable normally open trigger switch connected electrically in series with the solenoid coil and the source of AC electrical power, which is operable to physically interrupt the path of current flow to the solenoid coil at all times other than when the trigger is actuated by the operator.
It is an additional object of the present invention to provide a firing circuit which operates to delay the energization of the solenoid for a predeterminable period once the trigger has been activated.
It is a further object of the present invention to provide a firing circuit which operates to disable any further energization of the solenoid after the trigger has been released for a predeterminable time delay.
In accordance with the foregoing objects, the present invention provides a "one shot" firing circuit which includes a manually operable normally open trigger switch connected electrically in series with the solenoid coil and the source of alternating current electrical power for controlling the supply of electrical power to both the firing circuit and the solenoid, such that when the trigger switch is open, the path of current flow to the solenoid coil is physically interrupted. The firing circuit also includes controlled conduction means, such as an SCR, connected electrically in series with the trigger switch, the solenoid coil and the AC power source, for controlling the flow of current through the solenoid coil after the trigger switch has been closed. The firing circuit further includes a timing circuit connected to the gate of the SCR which is operable in response to the closing of the trigger switch to provide a predeterminable delay after the trigger switch is closed before enabling the SCR to conduct and permit a unidirectional current impulse to flow through the solenoid coil of sufficient magnitude to energize the solenoid. The timing circuit is also adapted to enable the conduction of the SCR upon a positive going zero crossing of a voltage waveform from the AC power source, such that a single positive polarity half cycle current waveform is applied to the solenoid coil. The timing circuit also operates to disable further unidirectional current impulses through the solenoid coil once the solenoid has been energized until a predeterminable delay after the trigger switch has opened.
Additional advantages and features of the present invention will become apparent from a reading of the Detailed Description of the Preferred Embodiment which makes reference to the following set of drawings in which :